I worked out the missing step.
During the Linux boot - in the serial terminal it prompts to press a key.
Press a key and this enters U-boot terminal
Then
run jh_mmcboot
etc.
Then this starts the Jailhouse DTB (Device Tree Blob)
Then I run type these commands...
insmod /lib/modules/4.14.98-imx_4.14.98_2.0.0_ga+g1175b59/extra/driver/jailhouse.ko
insmod /lib/modules/4.14.98-imx_4.14.98_2.0.0_ga+g1175b59/extra/driver/uio_ivshmem.ko
jailhouse enable /usr/share/jailhouse/cells/imx8mq.cell